The results of radiation inactivation studies have indicated that the insulin receptor in rat liver membranes is composed of at least two components: an affinity-regulatory component with an apparent molecular weight of about 300,000 and a binding component with a molecular weight of about 100,000. The affinity-regulator acts as an inhibitor of insulin binding by decreasing the affinity of the binding component for insulin. Conditions which alter the affinity of the receptor (pH and ionic strength) also alter the inactivation profile. The higher the affinity of the receptor, the larger the proportion of binding components which are not associated with affinity-regulators. The interaction between the affinity-regulator and binding component are reversible suggesting that these components are not covalently bound. The affinity-regulator is a trypsin-sensitive membrane protein which can be separated from the binding component by lectin-affinity chromatography. Insulin bound to the receptor causes a change in the size of the binding component from 115,000 to 76,000 with no change in the apparent size of the affinity regulator.